The present invention relates to a magnetic recording medium such as a magnetic tape, particularly, it relates to a magnetic recording medium with a magnetic layer formed by applying on a support a magnetic coating mainly containing a ferromagnetic powder and a binder. More specifically, it relates to a magnetic recording medium for high density recording, including a hexagonal ferrite dispersed in a binder in a magnetic layer, particularly preferable for use in a system using an MR head (magnetoresistive head) in reproduction.
Conventionally, as a magnetic recording medium such as a video tape, an audio tape, and a magnetic disc, those having a magnetic layer provided by coating on a support a ferromagnetic iron oxide, a Co modified ferromagnetic iron oxide, a CrO2, a ferromagnetic metal powder, a hexagonal ferrite, or the like dispersed in a binder are widely used. It is known that the hexagonal ferrite has the excellent high density recording characteristics among these examples as described, for example, JP-A-60-157719, JP-A-62-109226, and JP-A-3-280215(the term xe2x80x9cJP-Axe2x80x9d as used herein means xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d). JP-A-5-12650 discloses a method for improving the surface property, the short wavelength output, the erasure characteristic and the durability by adjusting a thickness of a magnetic layer containing the ferrite to 0.1 to 0.6 xcexcm and providing a non-magnetic layer thickener than the magnetic layer between the magnetic layer and a support. JP-A-5-225547 discloses a magnetic recording medium having a non-magnetic layer on a support and forming a 0.1 xcexcm or less thick magnetic layer containing a magnetic powder, which is excellent in high frequency characteristic, signal rewriting characteristic, and durability.
Moreover, JP-A-3-286420, IEEE. Trans. Mag, vol. 24, No. 6, Nov. 1988, p. 2850, or the like disclose the influence of the anisotropic magnetic field Hk of a hexagonal ferrite on the electromagnetic characteristics of a magnetic recording medium. Moreover, the former discloses a magnetic recording medium comprising two magnetic layers provided on a non-magnetic layer, wherein the lower magnetic layer has an axis of easy magnetization in the longitudinal direction, and the upper magnetic layer contains a magnetic powder having the anisotropic magnetic field of 3,000 Oe or less for providing a magnetic recording medium capable of providing a high output in a wide range from a long wavelength to a short wavelength. JP-A-8-115518 proposes a medium for high density recording having an Hc of 103.5 to 298 kA/m, an Hc/Hk of 0.30 to 1.0, and a squareness ratio SQ in the in-plane direction of 0.65 to 1.00. It is characterized by specifying each numerical range of the Hc, Hc/HK, and SQ in the in-plane direction of a magnetic layer containing a hexagonal ferrite powder. Thereby, the ultra short wavelength output necessary for high density recording is improved dramatically. However, since it has a high noise in the case of use by an MR head, it is not preferable.
Recently, a highly sensitive reproduction head (MR head) utilizing the magnetoresistance is used for a data recording system for computers, and the system noise depends on noise derived from the magnetic recording medium. Okabe, et al. suggests that use of a Ba ferrite medium in combination with an MR head is preferable since it can avoid saturation of the MR head (IEEE. Trans. Mag., Vol. 32(5), p. 3404-3406 (1996)). Development of finer ferromagnetic particles is promoted for reducing the medium noise. However, it is presumed that the stability of the magnetization transitional area becomes the issue to be tackled due to influence of the thermal fluctuation as the reduction of ferromagnetic particle size. The magnetization stability is evaluated by a formula of KuV/kT (wherein Ku represents a magnetic anisotropic constant, V represents a particle volume, k represents a Boltzmann constant, and T represents an absolute temperature). Concerning the particle volume and the thermal fluctuation of the metal tapes, there is a report by Toshiyuki Suzuki, et al. (Shingakugihou, MR97-55, P. 33-40, Nov. 21, 1997).
Since the hexagonal ferrite has a saturated magnetization of about ⅓ to xc2xd with respect to that of the ferromagnetic metal powder, it is difficult to make Ku large, and thus the thermal fluctuation is large. Furthermore, a magnetic recording medium using a hexagonal ferrite is said to have a large interaction between the particles so as to influence the medium noise level. Although the excellent magnetization stability is said to be provided owing to a large interaction between the particles, when particle is inverted magnetically for some reasons, magnetic substances in the vicinity may also be magnetically inverted. Therefore, a problem is involved in that it is difficult to sufficiently ensure the C/N in the case of reproducing with an MR head a medium for high density recording produced using a hexagonal ferrite magnetic powder with a minute particle size.
In order to solve the problems of the conventional techniques, the present invention is made paying attention to the inter-particle interaction of a magnetic recording medium using a fine hexagonal ferrite. Specifically, an object of the invention is to provide a magnetic recording medium having a magnetization stability superior to that of the conventional products, and good short wavelength output and C/N at the time of reproduction using an MR head.
The object of the invention is attained by a magnetic recording medium comprising a non-magnetic layer containing an inorganic non-magnetic powder dispersed in a binder provided on a support, and a magnetic layer containing a ferromagnetic powder dispersed in a binder provided on the non-magnetic layer, wherein the magnetic layer contains a hexagonal ferrite magnetic powder having an average tabular diameter of 10 to 28 nm and has a coercive force (Hc) of 135 to 400 kA/m , a ratio (Hc/Hk) of the Hc to an anisotropic magnetic field (Hk)of 0.3 to 0.6 and a maximum value of xcex94M of 0 to 0.10.
Moreover, according to the invention, it is preferable that the magnetic layer thickness is 0.01 to 0.5 xcexcm, and the residual magnetic flux densityxc3x97magnetic layer thickness is 5 to 100 mTxc2x7xcexcm.